Industry and trade are nowadays seeking high-performance pressure-sensitive adhesives which are used in the construction sector for structural bonding in the form of single-sided and double-sided adhesive tapes as well as in the form of transfer adhesives. Such high-performance pressure-sensitive adhesives are distinguished by improved creep strength against static mechanical stresses even at temperatures above 100° C.
State of the art are high-performance pressure-sensitive adhesives which, after application to the carrier material, are crosslinked in the solvent-free state by irradiation in the ultraviolet region. The disadvantage of such systems is that the irradiation of the pressure-sensitive adhesive layer takes place on one side. The intensity of the UV light is greatest at the surface of the pressure-sensitive adhesive layer, that is to say at the surface which later constitutes the adhesive surface. Accordingly, the crosslinking density is also greatest at that surface. The instant adhesion and the surface tackiness suffer as a result, which can lead to immediate or subsequent adhesive failure of the adhesive bond.
That is the case especially when pressure-sensitive adhesive layers having a layer thickness greater than 80 μm or 100 μm are irradiated with UV light. High-performance pressure-sensitive adhesives having layers greater than 100 μm are used especially where the substrate surface exhibits increased roughness.
According to EP 904 853, improvements in such systems can be achieved by selective UV irradiation, for example through a perforated mask or template. There result highly crosslinked regions having a high mechanical bearing strength and reduced tackiness, together with regions of lower crosslinking density but having good surface tackiness.
The outlay in terms of apparatus and the costs for UV irradiation in general, and especially for UV radiation using a perforated mask, can be considerable. A further disadvantage is also that the UV irradiation can only be carried out on the high-performance pressure-sensitive adhesive in tape form. An alteration of the mechanical properties at the bonding is not normally possible for various reasons.
A further solution for producing high-performance pressure-sensitive adhesives is thermal after-crosslinking of the adhesive layer. There result pressure-sensitive adhesives which exhibit good instant adhesion and acceptable creep resistance before thermal activation and have high strength and static bearing strength after activation. The long exposure times of from 10 to 30 minutes at high activation temperatures in the range from 120 to 150° C. are a disadvantage of systems used hitherto. Examples are heat-activatable HAF films (based on epoxy or phenolic resin, copolyamide with epoxy crosslinker, phenolnitrile rubber) from Tesa Industries, Beiersdorf AG (D-Hamburg) or the heat-curing adhesive tape SBT 9245 from 3M (USA-St. Paul Minn.), described in G. Bennet et al.; Strukturelles Haftklebeband—Eine Innovation in der Klebetechnik; Conference Volume 10. Int. Symposium Swissbonding, CH-Rapperswil, 1996, p. 197-205.
The field of application of systems used hitherto is limited by the long exposure times. Many substrates become damaged if they are exposed to temperatures in the range from 120 to 150° C. for from 10 to 30 minutes. For that reason, systems having lower activation temperatures and/or exposure times are desirable.
An overview of pressure-sensitive adhesives, and especially of structural pressure-sensitive adhesives that are after-crosslinkable by UV irradiation or thermally, is given by P. L. Geiss, in “Verarbeitungskonzepte und Belastungskriterien für Haftklebstoffe” Hinterwaldner Verlag, Munich, 1998.
In the as yet unpublished Application PCT/CH99/00577 of the same Applicant, a reactive system for the preparation of spontaneously crosslinking pressure-sensitive adhesives is described. That system consists of an aqueous dispersion containing at least one isocyanate-reactive polymer and at least one surface-deactivated solid isocyanate. Features of the said dispersions are                glass transition temperature Tg of the isocyanate-reactive polymer less than or equal to −5° C.        storage modulus G′ of the isocyanate-reactive polymer less than or equal to 107 Pa, measured at 10 Hz and 10° C.        minimum film-forming temperature MFT less than or equal to +5° C.        mean particle diameter of the solid isocyanates less than or equal to 10 μm.        
The object of the invention according to PCT/CH99/00577 is the preparation of aqueous dispersions or solutions containing solid, surface-deactivated polyisocyanates and isocyanate-reactive polymers, which are stable to storage in that form, that is to say in the form of aqueous dispersions or solutions, but in which, when applied in the form of a layer, partial crosslinking is initiated after removal of a substantial portion of the water.